Applying a magnetic field has been known as a method of controlling a magnetization direction of a magnetic material. In, e.g., a hard disk drive, a magnetization direction of the medium is reversed by a magnetic field generated from a magnetic head to execute a write-in. In a magnetic random access memory, a magnetization direction of a memory cell is controlled by applying a current-induced magnetic field generated by passing a current to write-in lines provided near a magnetoresistive effect element included by the memory cell. The magnetization direction control by external magnetic fields has a long history, and can be taken as established technology.
On the other hand, along with a recent progress in nanotechnology, bit or cell size of magnetoresistive effect devices has been reduced. Accordingly, magnetization control must be done locally on a nanoscale. However, localizing a magnetic field is difficult, because the magnetic field fundamentally spreads spatially. This causes a problem of “cross talk”. Even when a specific bit or cell is selected to control its magnetization direction, a magnetic field spreads to adjacent bits or cells and executes an incorrect write-in on the bits or cells. If a magnetic field generation source is made small to localize the magnetic field, sufficient magnetic fields cannot be generated.
A “current direct driving magnetization reversal method” has come to attract attention in order to avoid the above-mentioned problem (e.g., F. J. Albert, et al., Appl. Phy. Lett. 77, 3809 (2000)). In the method, a current is supplied to a magnetic layer of a magnetoresistive effect element to spin-polarize electrons. Spin-polarized electrons are caused to pass through a target magnetic layer to reverse its magnetization. More specifically, when an angular momentum of the spin-polarized electrons is transmitted to electrons of a magnetic material of the target magnetic layer, a magnetization direction of the magnetic material is reversed.
When the method is employed, by which the current can be caused to more directly act on the magnetic material, magnetic state of the magnetic material may be easily controlled on a nanoscale. The current for the magnetization reversal can also decrease according to miniaturization of the magnetoresistive effect element. Hence, the technique of the “spin-polarized current direct driving magnetization reversal” contributes to realizing spin electronics devices such as high-density hard disk drives or MRAMs.
A method of a spin torque switching assisted by an oscillating-field is disclosed in USPA0070047294. A magnetic memory according to the method is capable of addressing magnetic cells arrayed in a matrix with bit and word lines of the memory. Each magnetic cell is provided with a magnetoresistive effect element which has a layered structure consisting essentially of a ferromagnetic fixed layer/a tunnel barrier layer/a ferromagnetic free layer (magnetic recording layer), and is recorded according to a direction of current flowing across itself. Each magnetic cell is provided also with a switching element (transistor) connected to the magnetoresistive effect element in series. The magnetoresistive effect element of each magnetic cell is connected to a bit line. The switching element (transistor) is connected to a word line. Furthermore, the magnetic memory system has a DC power supply to send a DC current, and an AC power supply to generate an oscillating magnetic field at the time of recording.
The technique of the “spin-polarized current direct driving magnetization reversal” has a problem that an incomplete magnetization reversal often occurs, as a pulse width of a write-in current becomes short. The problem has been pointed out in a literature, e.g., Shehzaad Kakaa, et al., Journal of Magnetism and Magnetic Materials Vol. 286, (2005) p. 375. In order to avoid the problem, it is necessary to increase the write-in current more. An increase in the write-in current causes a further problem about reliability, i.e., characteristics of the magnetoresistive effect element deteriorates under an influence of heat generated within the element. In a magnetic memory using the spin-polarized current direct driving magnetization reversal method, the increase also leads to a problem being caused by miniaturizing selection transistors, i.e., by increasing memory capacity. Then, power consumption of the magnetic memory also increases to be a problem.
In such a magnetic memory described in USPA0070047294, it is not easy to fully flow a radio-frequency (RF) current having a frequency of several GHz or more through a single bit line or word line without a ground line in order to execute a write-in for the memory, causing a big problem.